This invention relates to current mirror circuits and amplifiers and other circuits incorporating current mirror circuits. More particularly, the present invention relates to circuits and methods for ensuring that the output current of a current mirror can be reduced to zero and to a variable-gain amplifier employing the current mirror circuits and methods to ensure that an input signal can be fully attenuated.
The operation of current mirror circuits, formed from two or more bipolar junction transistor devices coupled such that the base-emitter voltages of the two devices are equal, is well known. A desirable feature of such circuits is that an input current passing through one branch of the circuit is accurately reflected in an output current passing through a second branch. The accuracy of the circuit is typically facilitated by fabricating the two transistor devices on the same chip, where the intrinsic parameters of the two devices are nearly equal, such that the currents through the two devices have a substantially linear relationship, whereby each current is substantially proportional to the emitter area of the corresponding transistor.
Current mirrors are useful in a variety of circuits, including electronically controlled amplifiers and video faders. For example, such amplifiers and faders may operate by converting a control voltage to a control current. The control current is then reflected in a current mirror which acts as a current source to provide gain control. These amplifiers and faders provide accurate gain control because the reflected current follows the control voltage with accuracy.
In many applications, the control voltage is produced by an inexpensive operational amplifier which often has a small offset error. When the control voltage is at its minimum, the offset error may prevent the control current generated by the current mirror from being reduced to zero, resulting in inadequate attenuation of the signal passing through the amplifier. Thus, a serious side effect of accurate gain control in such an amplifier is that it can prevent the amplifier from providing the attenuation required to make a signal indiscernible.
Amplifier circuit designs using current mirrors are frequently based on variable-transconductance multiplier circuits which use cross-coupled differential stages. These multiplier circuits typically have two differential pairs of input transistors connected in parallel to input terminals where a first differential input voltage is applied. The outputs of the differential transistor pairs are cross-coupled. Each differential pair is coupled in series with one of a second pair of transistors which are coupled to receive a second differential input voltage. The magnitudes of the currents through the cross-coupled outputs differ as a function of the product of the first and second differential input voltages. The differential output current is used to produce an output voltage which is also a function of the product of the differential input voltages. When used as an amplifier, a variable-transconductance multiplier circuit usually has the first differential input voltage coupled to the gain control signal and the second differential input voltage coupled to the signal being amplified. Current feedback circuitry can be used to couple the output voltage to the second differential input voltage to control the maximum gain, as is well known in the art.
One limitation of this amplifier design is that the two cross-coupled differential pairs can control the gain of only a single differential input (due to the fact that the second differential input is coupled to the signal being amplified). This prevents the precise gain control derived from the amplifier from being easily applied to multiple-input circuits, such as video faders.
Another disadvantage of this type of amplifier circuit is that they typically consist of a single type of transistor device (such as an NPN-BJT) and are not well balanced for accurately reproducing signals whose polarity may invert. For example, negative polarity output voltages can be produced only by subtracting from one of the output currents either an average current signal or the other output current.
Furthermore, the gain of the input signal can be minimized only if the differential gain control input is precisely zero, which results in the input signal being multiplied by zero. Any error in the gain control input results in either a noninverted or inverted output of undesirable magnitude. Thus, even if two such amplifiers were coupled to inversely-related control signals to construct a fader, small errors in the differential gain control could prevent the amplifier from providing the required attenuation.
Errors in the differential gain control voltage often derive from offset errors in operational amplifiers that supply the gain control voltage. The differential gain control voltage is usually obtained by driving control currents across diode junctions. As described above, control currents are typically obtained from current mirror circuits which are themselves controlled by a voltage control signal. Because the voltage control signal is usually produced from an inexpensive operational amplifier (including the small unavoidable offset error), the current mirrors may prevent the magnitude of the differential gain control voltage from reaching its minimum. This results in inadequate attenuation of the signal passing through one of the amplifiers.
For example, in a typical video fader employing two amplifier circuits, the gain of one input signal may need to be reduced by factor of 1,000 (i.e., 60 dB) to prevent its image from being visible as the magnitude of a second input increases. If the linear control voltage operates in a full scale level of 0-2.5 V, the error must be less than 2.5 mV. This requirement is much better than inexpensive operational amplifiers can achieve without trimming.
Prior amplifiers often induce an offset error in the control circuitry to ensure that the control current can be reduced to zero at control voltage levels slightly greater than zero. This offset error can be produced in the current mirror which controls the amplifier gain in response to the control voltage. Gain errors are also often added to the current mirror circuit to adjust the realized control current function closer to the ideal function at higher current levels. A disadvantage of this approach is that the gain is distorted, such that the actual control current, and therefore amplifier gain, differ from their ideal levels at all but a single operating point.
In view of the foregoing, it would be desirable to provide a circuit and method for assuring that the output current of a current mirror can be reduced to zero when the input current falls below a predetermined error level.
It would also be desirable to provide a circuit and method for assuring that the output current of a current mirror accurately reproduces the ideal linear response over the majority of the range of operation.
It would be further desirable to provide an electronically controlled amplifier circuit having at least two input stages with current feedback, wherein the gain of each input is controlled by current steering.
It would be still further desirable to provide an electronically controlled amplifier with at least two current feedback input stages employing current steering, wherein complementary circuits are used to provide a more balanced response.
It would be even still further desirable to provide a circuit and method for controlling an accurate amplifier such that the gain is assured of being reduced to its minimum level when the control voltage falls below a threshold that is offset by a predetermined amount from its ideal minimum level.
Additionally, it would be desirable to provide a circuit and method for controlling an electronically controlled fader circuit, wherein the gain of each selected input is assured of being reduced to its ideal minimum level and the gain of the remaining inputs are assured of reaching the ideal maximum level when the control voltage reaches a corresponding extreme that is offset by a predetermined amount from the ideal extreme.